My understanding is that the only valid reason to turn down an ED acceptance is that you cannot afford to attend the university. If you run the NPC and it predicts that the university is going to be affordable, then I think that it is reasonable to expect it to be affordable. I would print out and save the NPC results. Then if the actual award ends up significantly worse than what the NPC predicted, you have a strong argument regarding why you backed out.
One daughter wanted to apply ED to a university where the NPC predicted that it would not be affordable – not even close. In that case we did not allow her to apply ED. However, if the NPC had predicted that it was going to be affordable we would have been okay with it.